As high-resolution analog to digital converters with a simple circuit structure, time analog to digital converters, referred to as TAD converters, have been conventionally developed. Examples of the TAD converters are disclosed in U.S. Pat. No. 5,396,247 corresponding to Japanese Unexamined Patent Publication No. H05-259907.
One typical example of the TAD converters of the U.S. patent publication includes a pulse delay circuit composed of a plurality of delay units that corresponds to a plurality of stages of delay. The delay units are serially connected to one another in a ring-like structure.
In the TAD converter, when a pulse signal is input to one of the delay units corresponding to the first stage of delay, a pulse signal is sequentially transferred by the delay units while being delayed thereby in the order from the first stage of delay units toward the last stage thereof. On the other hand, an analog voltage signal as a target for A/D conversion is input to each delay unit as power supply voltage, so that the delay time of each delay unit depends on the level of the power supply voltage (the analog voltage signal) supplied to each delay unit.
Specifically, the TAD converter is designed to:
count a number of stages (pulse delay units) through which the pulse signal has passed within a predetermined sampling period during circulation; this number of pulse delay units though which the pulse signal has passed within the predetermined sampling period depends on the level of the input analog voltage signal; and
obtain digital data corresponding to the level of the input analog voltage signal based on the counted number of stages (pulse delay units).
As described above, the TAD converter is designed to change the delay time of each delay unit based on the level of the input analog voltage signal, and to detect the level of the input analog voltage signal by counting the number of delay units through which the pulse signal has passed. Because the change in the delay time of each delay unit is nonproportional to that in the level of the input analog voltage signal supplied to each delay unit, the A/D converter output is not linearly changed depending on the change in the level of the input analog voltage signal. This causes the TAD to have nonlinear input-output characteristics, in other words, an input-output characteristic curve.
In view of the foregoing circumstances, methods of correcting the input-output characteristic curve of the TAD have been proposed. Examples of the correcting methods are disclosed in U.S. Pat. No. 6,891,491 corresponding to Japanese Unexamined Patent Publication No. 2004-274157.
One typical example of the correcting methods of the U.S. Patent Publication is configured to:
divide the input range of an analog voltage signal into a plurality of areas;
approximate, by a line, an input-output characteristic curve of a TAD within each of the divided input voltage areas;
derive a conversion equation for converting a coordinate point on the approximated line within each of the divided input voltage areas into a coordinate point on an ideal input-output characteristic line; and
correct the output digital value of the TAD with respect to each of the input voltage areas using the conversion equation.
Another typical example of the correcting methods of the U.S. Patent Publication is configured to:
obtain an M number of coordinate points by actually converting an input analog voltage signal into digital data with the use of a TAD;
express an input-output characteristic curve of the TAD by an n-th polynomial expression (n≦M−1); and
derive a conversion expression for converting the output digital values of the TAD into digital values on an ideal input-output characteristic line of the TAD based on the n-th polynomial expression.
Because the correcting methods set forth above need to derive the conversion expression for conversion of the output of the TAD, it may be desired to correct the output digital values of the TAD without using such conversion expressions, thereby converting them into digital values on an ideal input-output characteristic line of the TAD.